why there is no difference in render after denoise is on?
Let me give you a subtle hint of one possibility why this might happen:

What you may have noticed in the picture is that there are different sampling settings for the render and the viewport.

...but what could be the reason for having the same result with &quot;denoising&quot;on and off?

This might be when you take a clean image rendered with enough samples for it not to have much noise and try to denoise it. The difference is minimal in that case because the denoiser does not have much noise to remove. In your specific case the difference is there and I would not even call it that small. Let's actually look for the difference in the images you have provided for a minute: